Are the theories of relativity real?

[Qfwfq]

I fear you have missed my point. SR does not require isotropy of spacetime so your argument is not a problem with it. SR does suppose the world line of a particle to be timelike at each point, so that "before" and "after" have the same meaning for all observers. Where was I saying that time travel is possible? Certainly not locally, SR is consistent with observed causality, and the paradoxes of going back in time suggest that we shouldn't expect to find weird topologies either.

 

Whether spacetime "is real" is a subtle issue, starting with the semantic issue and getting into epistemology. However, the Lorentz transform shows that the geometry defined by the Minkowski metric is real. A Lorentz boost is a pseudo-rotation, i. e. a linear combination of one observer's space and time coordinates give those of the other. This would be impossible according to your opinion.

[CraigD]

…This is not an attack. I would like to understand more about this area.

Because the theory of relativity is so entrenched in scientific literature and terminology, and because skepticism and denial of its validity is so long-ongoing as to have developed its own community and publishing sub-industry, I’d like to take the closing line of this thread’s opening post in the spirit in which I believe it was written, and address that post’s questions in simplest and most unassuming manner of which I’m able. Although that post suggest confusion and misunderstanding about some common scientific terms (eg: “What evidence is there that the theories of relativity are actually laws?”), I’d like to put off discussion of them, and first address only theory explaining light, and how they and experiments testing their predictions lead to relativity.

 

Current and former students with a formal introduction to modern physics will likely recognize the following discussion – it’s essentially the one given to their students by most modern physics instructors prone to such discussion. As in those discussions, I appeal to the reader to set aside what you know, believe you know, have read or heard, etc., and follow the brief sequence questions, answers, definitions and logical conclusions.

 

We begin with the question “what is light (of what does it consist)?”

We answer if with two mutually incompatible answers, arguable the simplest rational one:

• “Ballistic” theory - Light consists of a stream of particles of some sort of material, similar to water streaming from the nozzle of a hose, or bullets from the barrel of a machine gun.
  
• “Wave” theory - Light is a disturbance in some sort of material, similar to waves on the surface of a pool of water, in hanging length of rope, or vibrations in air, water, or a solid. By convention, we’ll call this material “the ether”.
  

Each theory allows us to make testable predictions.

 

For these tests, let’s assume we have a portable light-emitting machine (eg: a flashlight) – the “Emitter” - and a “speed of light measuring machine” – the “Detector” - that will unerringly measure how the speed of light passing through it. For simplicity in visualizing it, consider it to be similar to the machines used to measure the average speed of a car completing a straight road course – Using effectively perfect clocks, a timekeeper at the start of the course notes the car’s starting time, and one at the end of the course notes its finishing time. An official divides the known start-to-finish distance by the time, giving a speed. For our machine, assume we calibrate it so that a particular experiment gives a measurement of “1”, then don’t change it. For simplicity, measure the direction component of velocities so that east is negative, west positive, and perform the experiments both with the Emitter due east (appears in the first column in the table that follows) and due west (appears in the second column) of the Detector. To keep our data easy to display, assume that we can move the Emitter and Detector very fast - “0.1” as measured by a machine calibrated the same as our SOL-measuring machine - either due east or west, or hold them still, for possible velocities -0.1 0 and 0.1.

 

Now we make predictions with our two theories. Here they are in a table:

    Velocity           Measurement
(relative to ether)     Predicted
Emitter   Detector   Ballistic  Wave
-0.1      -0.1        1          1.1
-0.1       0          0.9        1  
-0.1       0.1        0.8        0.9
0        -0.1        1.1        1.1
0         0          1          1  
0         0.1        0.9        0.9
0.1      -0.1        1.2        1.1
0.1       0          1.1        1  
0.1       0.1        1          0.9

Detector  Emitter
-0.1      -0.1        1          0.9
-0.1       0          0.9        0.9  
-0.1       0.1        0.8        0.9
0        -0.1        1.1        1
0         0          1          1  
0         0.1        0.9        1
0.1      -0.1        1.2        1.1
0.1       0          1.1        1.1  
0.1       0.1        1          1.1

From a theoretical and experimental point of view, we’re in excellent shape. The two theories give simple, and dramatically different predictions that can easily be tested given our experimental ability. Unfortunately, the experimental ability given in this example did not practically exist until the last decades of the 20th century.

 

Historically, though a gross oversimplification, these theories and predictions represent the state of scientific knowledge thought the end of the 19th century. Because in practical actuality, available emitter and detectors speeds are very small compared to the speed of light, and instruments for measuring the speed of light much more complicated and inaccurate than the machine in our example, the first compelling experimental test of these theories were not performed until 1887, via the famous Michelson–Morley experiment. Grossly simplifying this experiment’s complicated methods, tiny effect measured and problematic precision, here are its result, compared to our two sets of theoretical predictions:

    Velocity                   Measurement
(relative to ether)     Predicted       Actual
Emitter   Detector   Ballistic  Wave     
-0.1      -0.1        1          1.1        1
-0.1       0          0.9        1          1
-0.1       0.1        0.8        0.9        1
0        -0.1        1.1        1.1        1
0         0          1          1          1
0         0.1        0.9        0.9        1
0.1      -0.1        1.2        1.1        1
0.1       0          1.1        1          1
0.1       0.1        1          0.9        1

Detector  Emitter
-0.1      -0.1        1          0.9        1
-0.1       0          0.9        0.9        1
-0.1       0.1        0.8        0.9        1
0        -0.1        1.1        1          1
0         0          1          1          1
0         0.1        0.9        1          1
0.1      -0.1        1.2        1.1        1
0.1       0          1.1        1.1        1
0.1       0.1        1          1.1        1

In short, but certain terms, both theories fail. Though years of research hoping to explain away this failure followed, and continue on the fringes of science to this day, both the ballistic and ether wave theories of light came to be considered dead by mainstream science due of these experimental failures.

 

These experiments revealed a simple but profound (and controversial) scientific law: the speed of light in vacuum is independent of the velocity of its emitter or detector. (Commonly called “the invariance of c”)

 

Though I promised in my first paragraph to put off discussion of scientific terminology, some in necessary here. A scientific law – though not a consistently defined and used term – generally refers to a statement of apparent truth, such as the preceding. In an explanative sense, a law is poorer than a theory – it offers no explanation of why the truth is so, only the observation that it apparently is.

 

IMHO, no accepted theory compellingly explains the invariance of c. In particular, relativity does not explain it, only accepts it as a postulate before proceeding to predict various consequences of it and its other postulates. Time dilation is one of these predictions.

 

Having established some background for it, I’m prepared to address the opening post’s questions

What evidence is there that the theories of relativity are actually laws?

Several previous posts have provided links to experimental validation of this and other prediction of relativity, supporting the conclusion that these predictions unfailingly describe actual reality. However, as I discussed above, relativity is not a collection of laws, but a theory offering an explanation of empirically observed laws.

Are they applicable?

Yes. An astronaut scooting around space at .99 c really would age at about 1/7th the rate of one who did not, a dramatic and noticeable effect.

 

In terms of practical applicability, it’s unlikely (regrettably, for devoted science fiction readers and futurists like me) that a human being ever will scoot about space at .99 c. However, sub-atomic particles on and near Earth regularly do so under the affect of natural (eg: stars) and artificial (eg: particle accelerators) causes, and behave as predicted by relativity, though these effects are less dramatic, intuitive, and easy to accept as would be the effect on a space-scooting astronaut.

If you must assume certain conditions in the creation of a hypothesis without being able to reproduce the same conditions, then what have you proven?

If this were the case, you would be unable to experimentally confirm (“prove”) the theory. However, as mentioned in the preceding and several previous posts and linked-to sources, this is not the case. The conditions necessary to test the predictions of relativity are available naturally and artificially, and have been used to successfully test the theory.

What is the purpose of these studies [of relativity] if we are not able to use them?

Although not necessary for many engineering tasks (eg: building bridges), for applications involving the usually small, fast, and/or precise (eg: the GPS), relativity must be taken into account.

[Little Bang]

Excellent post Craig.

[jedaisoul]

SR is consistent with observed causality, and the paradoxes of going back in time suggest that we shouldn't expect to find weird topologies either.

We seem to be in agreement, at least so far as "local" space-time is concerned.

 

Whether spacetime "is real" is a subtle issue, starting with the semantic issue and getting into epistemology.

Yes, that is what I'm interested in. Everything I know about the epistemology of space-time tells me time is not a dimension (in the same sense as the spatial ones are). Material objects physically exist in the present. The past and the future do not exist physically, therefore time travel is a non-starter.

 

The idea of a four-dimensional space-time continuum is not wrong. It correctly tells you what would happen if time travel were possible. But it isn't, so what's the point of perpetuating the status quo?

 

However, the Lorentz transform shows that the geometry defined by the Minkowski metric is real.

I would paraphrase your statement to be "If the Lorentz transformation (as used by Einstein) is correct, it implies a real change in space-time geometry. I.e. A real dilation of time and a real contraction of distances. I agree that is true, but it's a big "if". If you agree that time travel is not possible (even if just locally), then do you agree that the effects of the Lorentz transformation (as described bt Einstein) are not possible (at least locally)?

 

And what does "locally" mean? In astronomic terms, our solar system is "local", the arm of the milky way that we are in is "local". Indeeed arguably the whole galaxy is "local" (compared to the vast expanse of the known universe). Exactly where does "local" end in your view?

 

Regards, Terry.

[jedaisoul]

IMHO, no accepted theory compellingly explains the invariance of c. In particular, relativity does not explain it, only accepts it as a postulate before proceeding to predict various consequences of it and its other postulates. Time dilation is one of these predictions.

 

Excellent point.

 

Yes. An astronaut scooting around space at .99 c really would age at about 1/7th the rate of one who did not, a dramatic and noticeable effect. In terms of practical applicability, it’s unlikely that a human being ever will scoot about space at .99 c.

 

I agree about it being unlikely that astronauts will ever travel at a significant proportion of the velocity of light, but I am not comfortable with the ageing bit. Please see below...

 

However, sub-atomic particles on and near Earth regularly do so under the affect of natural (eg: stars) and artificial (eg: particle accelerators) causes, and behave as predicted by relativity... The conditions necessary to test the predictions of relativity are available naturally and artificially, and have been used to successfully test the theory.

 

IMHO, the conditions necessary to prove the predictions of relativity do not exist. This is because the core of the predictions of SR is a difference in the passage of time and/or physical distances when viewed from differeing rest frames of reference. All the experiments and observations on sub-atomic particles are only observed from one rest frame of reference. Yes, what occurs in that frame of reference agrees with the predictions, but does that prove a difference with what occurs in the other frame of reference? What occurs there is assumed to match the predictions, so it does!!!

 

The crux is how reasonable is it to assume what takes place in the other frame of reference? I would suggest that the "proof" is far less clear cut than it might appear.

 

Regards, Terry.

[Qfwfq]

Everything I know about the epistemology of space-time tells me time is not a dimension (in the same sense as the spatial ones are).

What exactly, that you know?

 

Material objects physically exist in the present. The past and the future do not exist physically, therefore time travel is a non-starter.

 

The idea of a four-dimensional space-time continuum is not wrong. It correctly tells you what would happen if time travel were possible. But it isn't, so what's the point of perpetuating the status quo?

 

 

I would paraphrase your statement to be "If the Lorentz transformation (as used by Einstein) is correct, it implies a real change in space-time geometry. I.e. A real dilation of time and a real contraction of distances. I agree that is true, but it's a big "if". If you agree that time travel is not possible (even if just locally), then do you agree that the effects of the Lorentz transformation (as described bt Einstein) are not possible (at least locally)?

You seem to misconstrue SR as being based on the possibility of time travel. What "time travel" do you mean? It is quite certain that the proper time along paths that depart and meet again can differ, but this isn't "time travel" (and it is so much possible that it does happen).

 

And what does "locally" mean? In astronomic terms, our solar system is "local", the arm of the milky way that we are in is "local". Indeeed arguably the whole galaxy is "local" (compared to the vast expanse of the known universe). Exactly where does "local" end in your view?

To put it briefly, unless space-time is perfectly flat locally ends immediately, at zero distance! What it really means is "as described in differential terms".

 

Yes, what occurs in that frame of reference agrees with the predictions, but does that prove a difference with what occurs in the other frame of reference? What occurs there is assumed to match the predictions, so it does!!!

 

The crux is how reasonable is it to assume what takes place in the other frame of reference? I would suggest that the "proof" is far less clear cut than it might appear.

It's not a matter of "what happens in" but rather of "how it's described according to" different reference frames. There's no doubt about what the coordinate transformations are. Beyond that, you'd be talking about metaphysics.

[sanctus]

jedaisoul, I think there have as well been made some experiences in space with another frame of reference (I don't THINK that all satellites always were geostationary, but I'm not sure about that)...

 

having written this and re-read your post I think I understood you wrong.

Both reference systems can be and have been tested I think, take the muon decay (life-time in [imath]\mu s[/imath] if I remember well) when we measure a muon it is mainly as you said in our reference frame in which the muons time is dilated and hence it lives long enough so that we can measure it. I'm sure there is also a processus where muons are emitted in non-relativistic speeds and so you can verify the other frame...

[jedaisoul]

jedaisoul, I think there have as well been made some experiences in space with another frame of reference (I don't THINK that all satellites always were geostationary, but I'm not sure about that)...

 

I think you are referring to GPS satellites, which are not geo-stationary. They are adjusted for relativistic effects, but still need manual adjustment from time to time. Besides which, the motion of the satelite would have an effect under Newtonian relativity, but slightly different. The relativistic adjustment is very small (4.465 parts in 10^10), and most of it is gravitational rather than caused by motion. So I've been trying to obtain infomation on the relative magnitude of the manual adjustments. Without that it is impossible to know whether adjusting for Einsteinian rather than Newtonian relativistic effects (of motion) actually makes any difference.

 

having written this and re-read your post I think I understood you wrong. Both reference systems can be and have been tested I think, take the muon decay (life-time in [imath]mu s[/imath] if I remember well) when we measure a muon it is mainly as you said in our reference frame in which the muons time is dilated and hence it lives long enough so that we can measure it. I'm sure there is also a processus where muons are emitted in non-relativistic speeds and so you can verify the other frame...

 

No, the lifetime of the muon has not been measured in both frames of reference. Low energy muons have been measured in the laboratory and have an average lifetime of 2.2 u sec in our rest frame. The high energy muons bombarding the Earth have an average lifetime of 34.8 u sec in our rest frame (approximately 16 times the low energy figure). It is then asserted that a high energy muon has a lifetime in it's own rest frame of 2.2 (the figure found in the laboratory for low energy muons). This then "proves" that the relativistic effects of motion are real.

 

Now the difference in the lifetimes of the muons is impresive. There is cetainly a major difference between the lifetimes of low energy and high energy muons, but does that prove that it is caused by relativistic effects? Are we comparing like with like? I'm not so sure.

 

Regards, Terry.

[jedaisoul]

It's not a matter of "what happens in" but rather of "how it's described according to" different reference frames. There's no doubt about what the coordinate transformations are. Beyond that, you'd be talking about metaphysics.

 

It seems to me that the difference between "what happens in" and "how it's decribed according to" is directly relevant to the thread. The question was "Are the theories of relativity real"? That is a metaphysical question. So the question is whether the transformations represent real, actual, difference in the passage of time and spatial distances, or just abstract mathematical relationships? At least, thats how I interpret the question.

 

Regards, Terry.

[Erasmus00]

Now the difference in the lifetimes of the muons is impresive. There is cetainly a major difference between the lifetimes of low energy and high energy muons, but does that prove that it is caused by relativistic effects? Are we comparing like with like? I'm not so sure.

 

How can there be a fundamental difference between a high energy muon and a low energy muon? Energy (even in regular, Newtonian mechanics) depends on whose looking at the particle. Its observer dependent! Choosing between relativity, and a system where each observer has different, unrelated, fundamental parameters (i.e. muon lifetimes) should be easy.

-Will

[CraigD]

The question was "Are the theories of relativity real"? That is a metaphysical question. So the question is whether the transformations represent real, actual, difference in the passage of time and spatial distances, or just abstract mathematical relationships? At least, thats how I interpret the question.

I agree.

 

Further, I believe the correct interpretation of present best theory, which includes Special and General Relativity, is yes, the transformations give by these theories do represent real, actual, difference in the passage of time and spatial distances, not just abstract mathematical relationships, and that all good (ie: formally defined, independently reproduced, and controlled properly for observational precision) experimental evidence to date has failed to contradict the predictions of these theories.

 

Although my opinion is only informal and intuitive, I believe that, given greater experimental capability, the predictions of current theory will fail. Present theory is not ultimate. However, just as Relativity has not displaced classical mechanics in the domain of the sort of physics necessary for tasks like building bridges and landing vehicles on the Moon, I doubt future theories will displace Relativity in the domain of physics like explaining the behavior of muons, and neutrinos, and precise timings in spacecraft.

 

It is not productive, I think, to discard theory that makes useful and experimentally verifiable and verified predictions because we can foresee that it is not ultimate and complete.

[jedaisoul]

How can there be a fundamental difference between a high energy muon and a low energy muon? Energy (even in regular, Newtonian mechanics) depends on whose looking at the particle. Its observer dependent! Choosing between relativity, and a system where each observer has different, unrelated, fundamental parameters (i.e. muon lifetimes) should be easy.

-Will

 

Hi, Yes, it is true that you can make kinetic enegry appear and disappear according to the frame of reference, but the same is also true of gravity. It does not mean that the energy (or the gravitational potential) is actually appearing out of nothing and disappearing into nothing. The high energy muon has an enormous and real kinetic energy. That is why I question whether you are comparing like with like.

 

However appropriate you may cosider it to be to take two different samples of muons in different conditions and compare them, this inevitably involves an assumption being made. It may be the best that you can do in practice, but it is not equivalent (i.e. the the same in all respects) as measuring the lifetime of a given batch of muons in two different frames of reference.

 

Surely, if you could you would measure the lifetime of a batch of muons in two different frames of reference? Would that not avoid having to make an assumption of equivalence? Do you agree that is true?

 

Regards, Terry.

[jedaisoul]

It is not productive, I think, to discard theory that makes useful and experimentally verifiable and verified predictions because we can foresee that it is not ultimate and complete.

 

I agree. However that is not what I was suggesting. I have no disagreement with SR and GR as mathematical abstractions used as scientific tools. However, I'm suggesting that there are two aspects to physics:

a) The "how it works" of the cosmos.

B) The "what it is" of the cosmos.

 

What I'm not convinced of is the relevance of SR and GR to cosmology today. This is not a matter of whether they will ultimately be replaced, it's a matter of whether they are credible now. IMHO, GR is wholly inapropriate as a description of the "what it is" of the cosmos. It's a model that is built on the assumption that time travel is possible (though not necessarily feasible). It does not comply with causality.

 

When people like Inter ask questions about the reality of SR and GR, I think it is appropriate to point out these shortcomings, whilst stressing their immense value to science as mathematical abstractions.

 

Regards, Terry.

[Erasmus00]

Surely, if you could you would measure the lifetime of a batch of muons in two different frames of reference? Would that not avoid having to make an assumption of equivalence? Do you agree that is true?

 

The only assumption that has to be made is that all muons are the same. This seems quite reasonable. You seem worried about the fact that an assumption is being made, but the assumption is natural. If this assumption weren't made there would be no point in doing ANY physics, as the laws would vary wildly with circumstance. Nothing would be universal.

-Will

[CraigD]

Hi, Yes, it is true that you can make kinetic enegry appear and disappear according to the frame of reference, but the same is also true of gravity. It does not mean that the energy (or the gravitational potential) is actually appearing out of nothing and disappearing into nothing.

IMHO, this is exactly and in an Epistemologically very real way what all physics - since Galileo’s “System of the World”, and its eventual formalization into Newton’s ”Principles”, and continuing with Einstein’s Relativity - means. Kinetic energy (or gravitational potential energy) is entirely dependent on the inertial frame in which it is measured.

 

Let’s put aside the difficult-to-perceive and intuitively visualize body of a muon, and consider a more intuitive one, a baseball.

 

Imagine we are sitting side by side in the passenger seats of an automobile traveling down a highway at about 45 MPH (70 KPH). I gently toss you a baseball. The 0.145 kg contacts your hand with a speed of .5 m/s, converting its [math]\frac12 \cdot 0.145 \,\mbox{kg} \cdot (0.25 \,\mbox{m/s})^2 = 0.018125 \,\mbox{J}[/math] of kinetic energy into various forms (eg: mechanical motion of your hand and body, slight heating, a nearly inaudible “thump” sound)

 

Now imagine that I slightly misjudge my toss, and/or you your catch, and the ball sails out an open side window, where an unfortunate, unwitting experimental participant is standing waiting for a bus. Fortunately, it’s a chilly day, and this person is wearing a bulky coat that absorbs the ball’s [math]\frac12 \cdot 0.145 \,\mbox{kg} \cdot (20 \,\mbox{m/s})^2 = 29 \,\mbox{J}[/math] of kinetic energy, converting it into a muffled thump and a sudden velocity of about 0.85 m/s (1.9 MPH) in the direction the car is moving.

 

The difference between the 0.018 J “low energy” baseball and the 29 J “high energy” one was entirely due to the reference frame of the person who caught it. It was the same ball. It was no more “really”, intrinsically, low energy than high.

 

Effectively the same scenarios can occur with a naturally occurring cosmic proton/Earth’s atmosphere generated antimuon. In the first scenario, a recently created antimuon ([math]\mu^+[/math]) in the thin upper atmosphere interacts (“collides”) with another cosmic proton ([math]p^+[/math]) with a small-magnitude relative velocity, resulting in the muon being elastically deflected by a large angle. In the second, the antimuon collides at very high relative speed with many protons and electrons in atoms nearly stationary with respect to the earth, resulting in interesting, detectable interactions such as ionization and scattering that can reveal information about the structure of nucleons.

[CraigD]

What I'm not convinced of is the relevance of SR and GR to cosmology today. This is not a matter of whether they will ultimately be replaced, it's a matter of whether they are credible now. IMHO, GR is wholly inapropriate as a description of the "what it is" of the cosmos.

Given that much of conventional cosmology doesn’t require calculations involving massive bodies moving at relative velocities with magnitude a substantial fraction of the speed of light, or bodies with densities resulting in gravitational fields so intense that gravitational time dilation is pronounced, it’s practical to prefer the computationally easier formalism of classical mechanics to its relativistic extensions. However, this preference doesn’t, IMHO, render the relativistic formalism “inappropriate”, only unnecessary. The relativistic formalism doesn’t result in incorrect results, but results that differ from the classical ones by amounts far smaller than the range of observational error, while being far more difficult to calculate.

It's a model that is built on the assumption that time travel is possible (though not necessarily feasible). It does not comply with causality.

Formally, this is not true. Relativity is not derived from such an assumption. The possibility of causation violations – ie: closed timelike curves (CTCs) – is a derived consequence of the theory, not an assumption.

 

The assumption that causality is never violated is not required by any conventional scientific mechanical theory. Rather, it is an assumption of various philosophical ontologies, arguably originating with the Aristotelian school. The term cause and effect, and assumptions such as one by definition not preceding the other is more philosophical than scientific.

 

General (but not Special) Relativity does indeed imply the possibility of the existence of CTCs. However, this implication does not require their existence, nor that such curves are or are not significant on a cosmological or human scale.

 

If I may offer an inexact analogy, one may compare mathematical physics, CTCs, and the observed universe, to sociology, nuclear weapons, and the existence of the internet.

 

The invention of nuclear weapons in the mid 20th Century made possible the scenario of world wars so destructive that the technological culture that produced the discoveries making the creation of these weapons, and those later making the internet, possible, would cease to exists. Such a scenario was considered by many respected philosophers and scientists then and to this day not unlikely – the “Third world war fought with sticks and stones” of Einstein, Fermi’s implied reason for the apparent lack of widespread intelligence in the galaxy, and other “doomsday” predictions. However, the possibility of human self-annihilation is not the same as its inevitability.

 

Analogously, GR suggest that the universe might show obvious examples of CTCs, such as activists from the future appearing to kill Adolph Hitler or their own grandparents. The possibility is treated seriously enough that many respected science academics that resolutions of such paradoxes have been made using interpretations of scientific theories more fundamental than mechanics, such as the Many Worlds interpretation of quantum physics. However, the possibility of CTCs is not the same as the inevitability of them creating dramatic, detectable paradoxes.

 

PS: I believe this thread is more focused on the philosophical underpinnings and validity of scientific theories than the examinations of a particular theory or experiment. If there are no objections, I’ll move it to the Philosophy of Science forum.

[jedaisoul]

I believe this thread is more focused on the philosophical underpinnings and validity of scientific theories than the examinations of a particular theory or experiment. If there are no objections, I’ll move it to the Philosophy of Science forum.

 

I have no objection.